Carbon nanotubes (CNTs) are essentially hollow tubes, of a honeycomb wall structure made from solely carbon atoms. There is particular interest in single wall carbon nanotubes (SWNTs) as they show remarkable electrical and electronic properties, combining either metallic or semi-conducting behaviour with tuneable band gap, very low density and high mechanical performance. In order to create a macroscopic material preserving the unique properties of the individual carbon nanotubes, it would be necessary to avoid any intermediate processing steps, which often introduce topological defects and substantially shorten the length of individual nanotubes.
A successful continuous method of floating catalyst chemical vapour deposition (FCCVD) CNT synthesis uses various carbon sources (including ethanol, hexane and methane) and an iron catalyst source such as ferrocene. The continuity of the process relies on various important parameters which ensure uninterrupted synthesis. In this process, the reaction time for the generation of active species and their role in the formation of nanotubes takes approximately 3 seconds. In the case of process scale-up one requires to use higher furnace temperatures to guarantee sufficient energy transfer to the precursor substances compared to the time available in substrate growth setups. Furthermore the complexity of the process and rapid synthesis can lead to instabilities and formation of undesirable carbonaceous and metallic impurities, interfering with the realisation of macroscopic materials with the outstanding properties of individual CNTs.
Until now, using this method a sulphur compound has always been required to control the size of the iron catalyst particles. Sulphur is known to form stable bonds with iron, thereby demobilizing the catalyst on the surface and preserving the particle in its current size and guaranteeing a narrow size distribution of the iron nanoparticles. Iron atoms become available from their ferrocene precursor from about 400° C. and start to collide in the reactor tube. During the time until carbon becomes available for reaction, the growth of iron particles by coalescence of the atoms is uncontrolled and produces a variety of sizes in iron clusters. Due to its high stability, CH4 (methane) has a comparatively high pyrolization temperature and thus carbon only becomes available for reaction with the transition metal at around 1200° C. In order to keep these clusters small and in a narrow size range, sulphur compounds such as thiophene, carbon disulphide and others have been applied. These less stable hetero-compounds were chosen to release sulphur at temperatures similar to ferrocene pyrolization. Due to their high vapour pressure these sulphur compounds however are difficult to dose. Too much sulphur present for the reaction entirely encapsulates iron and thus hinders its catalytic activity completely. As a result, an excessive amount of very short, unaligned tubes are formed around the catalyst particles, commonly referred to as “impurities”. Moreover, sulphur compounds suitable for the continuous CNT production process generally show a high toxicity for humans, particularly the hazard of reproductive damage. Therefore, developing a process without any sulphur utilization is highly desirable.